1. Field of the Invention
The present invention relates to a light-emitting element in which a light-emitting layer containing an organic compound is interposed between a pair of electrodes, and a light-emitting device or a lighting device including the light-emitting device. The present invention also relates to a manufacturing method of the light-emitting element.
2. Description of the Related Art
In recent years, a light-emitting element (also referred to as an electroluminescent (EL) element) in which a light-emitting layer (hereinafter also referred to as an EL layer) containing an organic compound is interposed between a pair of electrodes has been actively developed. Lighting has attracted attention as one of application fields of the light-emitting element. This is because a lighting device including the EL element has features different from those of other lighting devices; for example, the lighting device including the EL element can be thin and light, and perform surface emission.
As described above, an EL element has a structure in which a light-emitting layer containing an organic compound is interposed between a pair of electrodes. Thus, emission from the light-emitting layer is extracted through at least one of the pair of electrodes. Accordingly, in general, at least one of the pair of electrodes in the EL element is formed using a conductive film having a property of transmitting visible light (a transparent conductive film).
However, the resistivity of a transparent conductive film is one or two orders of magnitude higher than that of metal which easily conducts electricity. In particular, in a large-area element that is used for a lighting purpose or the like, there is a problem in that variation in luminance of a light-emitting surface of the light-emitting element due to voltage drop is significant. Therefore, Patent Document 1 suggests that a metal layer formed using a substance whose resistance is lower than that of the transparent conductive film is provided between the transparent conductive film and the light-emitting layer containing an organic compound (see Patent Document 1).
Another problem caused by the use of a transparent conductive film is about light extraction efficiency. In an organic EL element, light is emitted in all directions from a light-emitting layer. Therefore, total reflection occurs at an interface between layers whose refractive indexes are different from each other, so that much light is confined in a light-emitting element. The confined light is reabsorbed every reflection and thus attenuated, whereby the light almost disappears. Since a transparent conductive film has a higher refractive index than an organic compound, it is said that in a light-emitting element that is not devised to improve the light extraction efficiency, half or more of light is not extracted and disappears. In view of the above, various measures to improve the light extraction efficiency of an organic EL element have been taken (see Patent Document 2).